6,8-dimercaptooctanoic acid is the reduced form of α-lipoic acid, a coenzyme occurring in mitochondria, and has an ability of restoring reduced forms of glutathione or vitamin C back from their oxidized forms. However, 6,8-dimercaptooctanoic acid is so unstable in the air that it is oxidized back to α-lipoic acid.
A group of α-lipoylamino acids are described in Japanese Patent Publication No. S42-1286 (corresponding U.S. Pat. No. 3,238,224), in which glycine, methionine, glutamic acid, valine or the like, respectively, is bonded to α-lipoic acid.
An imidazole salt of α-lipoylaminoethylsulfonic acid is described in Example 24 in Japanese Patent Application Publication No. 2000-169371.
A pharmaceutical preparation for external use containing a lipoamide as an active ingredient is disclosed in Japanese Patent Application Publication No. S63-8316.
Other lipoic acid derivatives such as lipoyl esters are also known (Biochem. J. (1990) 271, 45-49).
Further, metal derivatives of dihydrolipoic acid and dihydrolipoamide are also known (Inorganica Chimica Acta, 192 (1992) 237-242)(J. Org. Chem. 1985, 50 2522-2524).
In the abovementioned situation, the present inventor found as a result of repeated studies that 6,8-dimercaptooctanoic acid metal chelate compounds, derivatives thereof and pharmacologically acceptable salts thereof have a melanin eliminating effect, i.e., that simple external application of the present compound to the skin under which aggregated melanin pigment is deposited, eliminates melanin without giving any damage to the skin, thus completed the present invention.
The present invention provides a melanin eliminator preparation comprising a 6,8-dimercaptooctanoic acid metal chelate compound, a derivative thereof or a pharmacologically acceptable salt thereof.